Eukaryotic cells divide by a directed, step-wise process referred to as the cell cycle. Cells must first replicate their DNA in S phase before separating their sister chromatids in mitosis (karyokinesis) and splitting off into two daughter cells (cytokinesis). In mammalian cells, DNA replication must be initiated at multiple sites (replication origins) throughout the genome to ensure that all the genetic material is duplicated prior to mitosis. To maintain genome integrity, DNA must be replicated only once per cell cycle, and so this process is highly regulated and governed by checkpoints. Before replication is initiated, origins must be licensed through the formation of pre-replication complexes (pre-RCs) in early G1.
Formation of pre-RCs involves the step-wise binding of the origin recognition complex (ORC) to origins followed by the binding of the loading factors Cdc6 and Cdt1. These proteins then recruit the putative DNA replicative helicase complex, MCM2-7. Once this pre-RC is formed, replication initiation requires the activation of S-phase-promoting serine/threonine kinases, Cyclin/Cdks and Cdc7/Dbf4. These kinases consist of an enzymatic subunit (CDKs and Cdc7) and a regulatory sub-unit (Cyclins for CDKs; Dbf4 or Drfl for Cdc7). They phosphorylate multiple MCMs in pre-RCs in a sequential manner, thereby activating the helicase and recruiting other DNA replication factors (Cdc45, GINS complex, etc.) for DNA synthesis (for reviews, see Kim et al., 2003; Kim et al., 2004; Lau et al., 2006; Lau et al., 2007; Stillman, 2005). MCM2 Serine-40 and Serine-53 are well-characterized phosphorylation sites for Cdc7/Dbf4 (Cho et al., 2006; Montagnoli et al., 2006; Tsuji et al., 2006).
Inhibiting regulators of replication initiation, such as Cdc6, Cdc7/Dbf4 or Cdc7/Drf1, has lethal consequences in cancerous cells, whereas normal cells are able to arrest and resume normal divisions once initiation activity is restored (Feng et al., 2003; Montagnoli et al., 2004; see Lau and Jiang, 2006, for review). Small molecule inhibitors of the protein kinase Cdc7 are thus attractive candidates for therapeutic intervention in cancer, inflammation and other cell proliferative disorders.